On Tue 26-11-19 22:25:27, Yafang Shao wrote: > On Tue, Nov 26, 2019 at 9:16 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > On Tue 26-11-19 08:02:49, Yafang Shao wrote: > > > There's one case that the processes in a memcg are all exit (due to OOM > > > group or some other reasons), but the file page caches are still exist. > > > These file page caches may be protected by memory.min so can't be > > > reclaimed. If we can't success to restart the processes in this memcg or > > > don't want to make this memcg offline, then we want to drop the file page > > > caches. > > > The advantage of droping this file caches is it can avoid the reclaimer > > > (either kswapd or direct) scanning and reclaiming pages from all memcgs > > > exist in this system, because currently the reclaimer will fairly reclaim > > > pages from all memcgs if the system is under memory pressure. > > > The possible method to drop these file page caches is setting the > > > hard limit of this memcg to 0. Unfortunately this may invoke the OOM killer > > > and generates lots of misleading outputs, that should not happen. > > > > I disagree that the output is misleading. Quite contrary, it provides a > > useful lead on the unreclaimable memory. > > > > We can show the unreclaimable memory independently, rather than print > the full oom output. > OOM killer is used to kill process, why do we invoke it when there's > no process ? > What's the advantage of doing it ? Consistency. > > > One misleading output is "Out of memory and no killable processes...", > > > while really there is no tasks rather than no killable tasks. > > > > Again, this is nothing misleading. No task is a trivial subset of no > > killable task. I do not see why we should treat one differently than the > > other. > > > > No killable tasks means there's task and the OOM killer may be invoked. > While no tasks means the OOM killer is useless. I disagree. > > > Furthermore, > > > the OOM output is not expected by the admin if he or she only wants to drop > > > the cahes and knows there're no processes running in this memcg. > > > > But this is not what hard limit reduced to 0 really does. No matter > > whether there is some task or not. It simply reclaims _all_ the memory > > as explained in other email. > > > > Are there any way to reclaim page cache only ? > No. Correct. And in absence of a solid usecase then I do not see a reason to add this. We have a global knob to achieve this and it has turned out to be abused and just used incorrectly most of the time. > I know it will relcaim all the memory. > If you really think this expression is a prolem, but does it > improtant that we should distingush between caches (both page caches > and kmem) and _all_ memory, especially when there's no processes ? I do not think we should distinguish different memory types and treat them differently when applying hard limit. -- Michal Hocko SUSE Labs
